Resveratrol

Yeast has taught us a great deal about the mechanisms of aging. But what about using yeast to fight the aging process itself?

A group of young scientists is trying to genetically engineer brewer’s yeast to make resveratrol, an antioxidant compound that activates sirtuins and may or may not extend mammalian lifespan (link):

A team of researchers at Rice University in Houston is working to create a beer that could fight cancer and heart disease. Taylor Stevenson, a member of the six-student research team and a junior at Rice, said the team is using genetic engineering to create a beer that includes resveratrol, the disease-fighting chemical that’s been found in red wine.

This isn’t the usual sort of thing we cover at Ouroboros. It’s a pre-pre-publication story from a decidedly non-scholarly source (ComputerWorld); and the article itself was written for such a general audience that it’s impossible to tell exactly what’s being done. Furthermore, the piece makes some bizarre claims (emphasis mine):

The students, using their own Dell, Lenovo ThinkPad and Gateway laptops, are now in the process of developing a genetically modified strain of yeast that will ferment beer and produce resveratrol at the same time. Stevenson said that as the research advances, the team will need to use one of Rice University’s computer grids to run compute-heavy genetic models.”

Really? What possible ramification of expressing a few synthases in yeast could require so much power that the researchers need to turn to grid computing? Ah well, these are the foibles of the lay press. And, possibly, the foibles of telling reporters from tech magazines the sort of thing they want to hear so that they cover your honors project.

So far, so good, but hardly surprising: file under “exercise is good for you, item #68232″. The interesting bit is that the mitochondrial and other changes are very similar to the physiological consequences of calorie restriction (CR), an intervention that is known to extend lifespan in model organisms and to delay age-related disease in humans. The authors argue that exercise may promote longevity through the same pathways as CR.

This fits in nicely with recent observations connecting exercise and CR: for example, resveratrol, thought to be a CR mimetic, improves exercise tolerance in mice, consistent with the idea that exercise and CR have something in common.

The next obvious question: Do exercise mimetics also promote longevity, and if so, do they do so by the same mechanism as CR?

Resveratrol, a natural product derived from grape skins and other plant sources, is widely (but not universally) believed to be an activator of the longevity assurance genes known as sirtuins. Despite some debate about its mechanism of action, the compound has received a great deal of attention as a possible pharmaceutical remedy for diseases of aging such as late-onset diabetes.

Most famously, resveratrol has been reported to increase the median lifespan of mice fed a high-fat diet, but that study has been subject to numerous criticisms. The diet in question was so unhealthy it would have made Morgan Spurlock blush, raising questions about its fairness as a model even for the most deranged Western diet. Furthermore, the quantity of resveratrol administered to the mice in the study corresponded to something like 1000 bottles of red wine per day. A skeptical reader could fairly claim that such a study, in which ridiculously high doses of a compound have an effect on an obscenely unhealthy animal, teaches us exactly nothing about what manageable doses of the same compound might accomplish in reasonably healthy people (which is, arguably, the point).

So: do manageable doses of resveratrol have health benefits — specifically, with respect to diseases of aging or aging itself? The first evidence in the affirmative has recently been published by Barger et al., who demonstrate that mice eating a normal ad libitum calorie-controlled* diet supplemented with resveratrol (at a much lower dose than in previous studies) undergo many of the same transcriptional changes as animals undergoing caloric restriction (CR):

Resveratrol in high doses has been shown to extend lifespan in some studies in invertebrates and to prevent early mortality in mice fed a high-fat diet. We fed mice from middle age (14-months) to old age (30-months) either a control diet, a low dose of resveratrol (4.9 mg/kg per day), or a calorie restricted (CR) diet and examined genome-wide transcriptional profiles. We report a striking transcriptional overlap of CR and resveratrol in heart, skeletal muscle and brain. Both dietary interventions inhibit gene expression profiles associated with cardiac and skeletal muscle aging, and prevent age-related cardiac dysfunction. Dietary resveratrol also mimics the effects of CR in insulin mediated glucose uptake in muscle. Gene expression profiling suggests that both CR and resveratrol may retard some aspects of aging through alterations in chromatin structure and transcription. Resveratrol, at doses that can be readily achieved in humans, fulfills the definition of a dietary compound that mimics some aspects of CR.

In addition to altered gene expression, the resveratrol-treated mice also exhibit delays in aging parameters (cardiovascular, endocrinological, metabolic) comparable to those caused by CR. The physiological and gene expression changes are observed in multiple tissues; taken together, they strongly support the hypothesis that resveratrol acts as a CR mimetic. Based on patterns of SIRT1 activity, however, the authors conclude that a subset of these changes (specifically, the delay in age-related cardiac decline) are not due to activation of SIRT1 by resveratrol.

The next question: Given that resveratrol and CR stimulate similar transcriptional changes, and that resveratrol yields some of the same physiological benefits as long-term CR, does low-dose resveratrol also have a favorable effect on median or maximum longevity? Based on these findings, I know how I’d bet, but for the ultimate answer, we’ll have to wait for the next paper.

The authors show once again that SIRT1 levels are low in the lungs of smokers, and that these levels diminish further in response to cigarette smoke. Smoke-exposed lung cells and macrophages exhibited increased inflammatory signaling (assayed primarily by looking at IL-8), as do cells in which SIRT1 has been artificially knocked out. Consistent with this, the RelA subunit of the master inflammation regulator NF-κB is hyperacetylated — recall that SIRT1 is a deacetylase — and consequently more active. Conversely, overexpression of SIRT1 decreases inflammatory cytokine production, echoing earlier results that described SIRT1 activation by resveratrol treatment.

Thus, a key longevity-assurance gene is also involved in restricting inflammation, which is a risk factor for some of smoking’s worst complications — chronic obstructive pulmonary disease (COPD) — as well as tumorigenesis. Tobacco is a major public health issue, so it’s not surprising that SIRT1 is getting attention in this context, but there is reason to believe that the phenomenon is general to organs other than the lung — e.g., see this review from Salminen et al., which describes how signaling from SIRT1 and FoxO transcription factors (mammalian homologs of the worm longevity gene DAF-16) can inhibit NF-κB signaling in a variety of systems. The authors close the circle by discussing the connection between longevity assurance and the mitigation of one specific age-related phenotype, inflammaging.

COPD is a leading cause of death worldwide, and it arises not only in smokers but in members of their households, as well as people exposed to environmental pollutants. A host of other inflammatory diseases, such as arthritis, beset the elderly and decrease quality of life. Increasing evidence that SIRT1 activity could mitigate the health harms of runaway inflammation point to even more potential uses for the new class of sirtuin-activating drugs that are currently under consideration as therapies against diabetes and metabolic syndrome.

The objective of this study was to determine the effects of aging and exercise training on SIRT1 activity and to identify a pathway linking SIRT1 to antioxidant response and cell cycle regulation in rats. SIRT1 is a NAD+-dependent deacetylase involved in the oxidative stress response and aging. The effects of aging and of moderate and prolonged exercise training in rats are unknown. We measured SIRT1 activity in heart and adipose tissue of young (6 months old), sedentary old (24 months), and trained old (24 months) rats using an assay kit. … Aging significantly reduced SIRT1 activity in heart, but not in adipose tissue, increased TBARS and 4-HNE and decreased Mn-SOD and catalase expression in both heart and adipose tissue. Aging did not affect FOXO3a protein expression in the heart or FOXO3a mRNA in adipose tissue. Exercise training significantly increased FOXO3a protein in the heart and FOXO3a mRNA in adipose tissue of aged rats. It also significantly increased Mn-SOD and catalase levels in both heart and adipose tissue. … We concluded that exercise training, which significantly increases SIRT1 activity, could counteract age-related systems impairment.

Note that the effects are observed both in cardiac tissue, where SIRT1 levels decrease with age, and in adipose, where SIRT1 activity remains constant throughout the lifespan.

These findings are consistent with recent observations that resveratrol induces the heat shock response, leading some to speculate that the compound exerts some of its positive effects via hormesis. Exercise is the quintessential example of beneficial hormesis: stress the body acutely now to make it more resistant to chronic stress later. Could resveratrol and exercise (and for that matter, calorie restriction) converge on a common pathway that confers stress resistance?

For those of you who lack the hepatic fortitude to consume 1000 bottles of red wine, I’m betting that New Year’s resolution to get off your duff and hit the gym is looking a lot more appealing.

Last year, we learned that the sirtuin activator resveratrolextends the healthspan of mice and increases exercise tolerance. Resveratrol occurs naturally in several plants, most famously the skins of red grapes; unfortunately for the would be life-extensionist, a human would have to consume upwards of 1000 bottles of red wine in order to approach the dose of resveratrol used in the rodent studies. What we needed was an orally bioavailable, clinically useful drug with the same specificity but much higher activity.

One year later, a collaboration between the pharmaceutical company Sirtris and the research group of David Sinclair (who co-founded Sirtris, and whose lab was responsible for the observation that resveratrol extends the lifespan of mice eating an unhealthy diet) has resulted in the development of sirtuin activators that are a thousand times more efficacious than resveratrol (link). While longevity data is not yet forthcoming, the compounds do have a significant influence on glucose homeostasis, and are being touted as a potential prophylactic or therapy against type II diabetes:

Small molecule activators of SIRT1 as therapeutics for the treatment of type 2 diabetes

Calorie restriction extends lifespan and produces a metabolic profile desirable for treating diseases of ageing such as type 2 diabetes. SIRT1, an NAD+-dependent deacetylase, is a principal modulator of pathways downstream of calorie restriction that produce beneficial effects on glucose homeostasis and insulin sensitivity. Resveratrol, a polyphenolic SIRT1 activator, mimics the anti-ageing effects of calorie restriction in lower organisms and in mice fed a high-fat diet ameliorates insulin resistance, increases mitochondrial content, and prolongs survival. Here we describe the identification and characterization of small molecule activators of SIRT1 that are structurally unrelated to, and 1,000-fold more potent than, resveratrol. These compounds bind to the SIRT1 enzyme–peptide substrate complex at an allosteric site amino-terminal to the catalytic domain and lower the Michaelis constant for acetylated substrates. In diet-induced obese and genetically obese mice, these compounds improve insulin sensitivity, lower plasma glucose, and increase mitochondrial capacity. In Zucker fa/fa rats, hyperinsulinaemic-euglycaemic clamp studies demonstrate that SIRT1 activators improve whole-body glucose homeostasis and insulin sensitivity in adipose tissue, skeletal muscle and liver. Thus, SIRT1 activation is a promising new therapeutic approach for treating diseases of ageing such as type 2 diabetes.

Standard qualifications: humans and mice have quite different metabolic needs, and it remains to be seen whether the drugs will work in humans. Even in the rodent, I’ll want to see next year’s paper (by that time, there should be lifespan curves available for animals that have taken the compounds for a long periods of time) before getting too terribly excited about the prospects of the first longevity drugs. It’s also important to keep in mind that the effect of long-term systemic sirtuin activation is unknown, and may even be harmful in certain key tissues (like the brain). In other words: I retain my skepticism; nonetheless, for the rest of this post I’m going to take these results at face value and look toward the future.

The work represents the culmination of a huge amount of progress in a relatively short time: in less than 15 years, the sirtuin story has evolved from basic biology in the simplest model organisms, through exhaustive (though essential) testing in larger animals, into a source of potential therapies for a major human disease.

Furthermore, for the first time we have a clearly defined path toward the regulatory approval and widespread use of a compound that could be used as a frank anti-aging drug. There are significant practical barriers to testing a longevity-enhancement therapy, not least of which is the timescale of the necessary studies. There are also institutional barriers: despite the inefficiency of treating every disease of aging separately, there’s still major reluctance on the part of funding and regulatory agencies to see aging as a disease per se (though even over my relatively short career in biogerontology, I have seen this changing for the better).

But a drug for which a clear clinical indication existed, shown to be efficacious against a widely acknowledged disease, could pass over regulatory hurdles and enter the clinic much more smoothly. Since clinicians could point to a specific short-term benefit of the drug, public acceptance (sometimes curiously hard to achieve in discussions of explicit longevity enhancement) might also come more readily. (One question: in advertisements, would the manufacturer have to warn patients that the drugs “may slow aging and extend the lifespan”?)

Hormesis in aging is represented by mild stress-induced stimulation of protective mechanisms in cells and organisms resulting in biologically beneficial effects. Single or multiple exposure to low doses of otherwise harmful agents, such as irradiation, food limitation, heat stress, hypergravity, reactive oxygen species and other free radicals have a variety of anti-aging and longevity-extending hormetic effects. Detailed molecular mechanisms that bring about the hormetic effects are being increasingly understood, and comprise a cascade of stress response and other pathways of maintenance and repair. Although the extent of immediate hormetic effects after exposure to a particular stress may only be moderate, the chain of events following initial hormesis leads to biologically amplified effects that are much larger, synergistic and pleiotropic. … Healthy aging may be achieved by hormesis through mild and periodic, but not severe or chronic, physical and mental challenges, and by the use of nutritional hormesis incorporating mild stress-inducing molecules called hormetins. The established scientific foundations of hormesis are ready to pave the way for new and effective approaches in aging research and intervention.

The discussion is quite broad, with appropriate emphasis given to classical examples of hormesis (radiation, thermal stress, and the emerging idea that calorie restriction is a form of hormetic stress) as well as forays into unusual stresses such as hypergravity.

At the end of the abstract he mentions “hormetins” — this is a fairly new term that has yet to gain substantial traction in the field, referring to compounds that confer stress resistance and possibly increased longevity by inducing low levels of stress themselves. As we learned recently, resveratrol may be among them.